Process for production of conductive catalyst particles, process for production of catalyst electrode capable of gas diffusion, apparatus for production of conductive catalyst particles, and vibrating apparatus

ABSTRACT

A process for production of conductive catalyst particles, a process for production of a catalyst electrode capable of gas diffusion, an apparatus for production of conductive catalyst particles, and a vibrating apparatus. The process can effectively and uniformly coat the particles of a conductive powder with a catalytic substance.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 10/477,840 filed on Nov. 17, 2003 which was the National Stageof International Application No. PCT/JP02/04726, filed on May 16, 2002,and which claims priority to Japanese Patent Document Nos. P2001-148613filed on May 18, 2001; and P2002-120822 filed on Apr. 23, 2002, thedisclosures of which are herein incorporated by reference.

BACKGROUND

The present invention relates to a process for production of conductivecatalyst particles, a process for production of a catalyst electrodecapable of gas diffusion, an apparatus for production of conductivecatalyst particles, and a vibrating apparatus.

It has been usual practice to produce a catalyst electrode capable ofgas diffusion from catalyst particles composed of carbon powder (as aconductive powder) and platinum (as a catalyst) supported thereon, incombination with a water-repellent resin (such as fluorocarbon resin)and an ionic conductor, by forming them into a sheet, for example, asdisclosed in Japanese Patent Laid-open No. Hei 5-36418 or applying themonto a carbon sheet.

The electrode thus produced may be used as an electrode for hydrogendecomposition as a constituent of a fuel cell of solid polymer type orthe like. In this case, the catalyst (such as platinum) ionizes fuel,giving rise to electrons, which flow through the conductive carbon. Thecatalyst also ionizes hydrogen, giving rise to protons (H⁺), which flowinto the ionic conducting membrane through the ionic conductor. Theseactions need interstices for passage of gas, carbon that conductselectricity, an ionic conductor that conducts ions, and a catalyticsubstance to ionize fuel and oxidant.

A typical way to make carbon powder (as a conductive powder) supportplatinum (as a catalytic substance) thereon is by dipping carbon powderin a solution containing platinum (in the form of ions), which isfollowed by reduction and thermal treatment. The processed carbon powdercarries platinum fine particles on the surface thereof, for example, asdisclosed in Japanese Patent No. 2879649.

The conventional method mentioned above, however, has the disadvantageof requiring the steps for reduction and thermal treatment. With thermaltreatment at an inadequately low temperature, it renders platinum poorin crystallinity, which results in mediocre catalytic characteristics.

Moreover, the fact that the carbon powder and the ionic conductor needto be in contact with each other because the catalytic substance likeplatinum ionizes fuel to give electrons, which flow through theconductive carbon, and also ionizes hydrogen to give protons (H⁺), whichreach the ionic conducting membrane through the ionic conductor, makesit necessary to apply the ionic conductor to the carbon powder on whichplatinum has been supported. Unfortunately, platinum (as a catalyticsubstance) functions only at the part which is in contact with gas, andconsequently, it becomes unfunctional when it is isolated from gas bythe ionic conductor.

There is an alternative method, which consists of coating carbon powderwith an ionic conductor and then causing the coated carbon powder tosupport platinum. This method has the disadvantage of requiring thermaltreatment to improve the crystallinity of platinum. However, thermaltreatment at a temperature sufficiently high for this purposedeteriorates the ionic conductor which is usually poor in heatresistance.

FIG. 11A is a schematic sectional view showing a conductive catalystparticle (produced by the conventional method) which consists of acarbon particle (conductive powder 1) and platinum particles (catalyticsubstance 18) supported thereon. Also, FIG. 11B is a schematic sectionalview showing a conductive catalyst particle, in which the carbonparticle is coated with an ionic conductor 19 and the platinum issupported thereon.

It is obvious from FIG. 11A that the conductive catalyst particlessupport on the surface thereof platinum in spherical form which hasprecipitated from the liquid phase. These platinum particles readilyseparate from the surface of carbon powder. Moreover, production in thismanner requires a relatively large amount of platinum. In addition,platinum in spherical form performs its catalytic function only on itssurface but does not function inside. Therefore, it has a low catalyticefficiency for its quantity. Another problem is that platinum enterspores in the surface of the carbon powder. (This is not shown.)

Further, causing the carbon powder to support platinum after applicationof the ionic conductor 19, as shown in FIG. 11B, necessitates thermaltreatment to improve the crystallinity of platinum. Unfortunately, theionic conductor 19 is usually poor in heat resistance and subject todeterioration upon heating at a temperature high enough for thedesirable crystallinity of platinum.

A catalytic electrode capable of gas diffusion which efficiently workswith a small amount of catalyst is disclosed in Japanese PatentApplication No. 2000-293517.

As disclosed, physical vapor deposition, such as sputtering as shown inFIG. 12, makes a catalytic substance adhere to the surface of theconductive powder 1.

However, the production process as disclosed therein consists ofdepositing the catalytic substance only on the surface of the conductivepowder 1 by physical vapor deposition as shown in FIG. 12. Deposition inthis manner, it is believed, takes place only on the surface of theconductive powder 1 lying in the uppermost layer in the container 4.Therefore, uniform deposition of the catalytic substance on the entireconductive powder placed in the container encounters difficulties.

A need therefore exists to provide improved processes and apparatusesfor producing conductive catalyst particles.

SUMMARY

The present invention provides a process for production of conductivecatalyst particles, a process for production of a catalyst electrodecapable of gas diffusion, an apparatus for production of conductivecatalyst particles, and a vibrating apparatus, wherein the processpermits the catalytic substance to be uniformly deposited on all of theconductive powder.

The present invention is directed to a process for producing conductivecatalyst particles by causing a catalytic substance to adhere to thesurface of conductive powder by physical vapor deposition whilevibrating the above-mentioned conductive powder, wherein theabove-mentioned conductive powder undergoes vibration together with avibration amplifying means arranged on a vibrating plane. The presentinvention is directed also to an apparatus used for the process forproducing conductive catalyst particles according to the presentinvention. This apparatus includes means to vibrate conductive powder,means for physical vapor deposition to cause a catalytic substance toadhere to the surface of the conductive powder, and means to amplifyvibration.

The process for production of conductive catalyst particles includes astep of vibrating a conductive powder together with balls and a step ofcausing a catalytic substance to adhere to the surface of the conductivepowder by physical vapor deposition.

The apparatus for production of conductive catalyst particles includesan approximately flat container to hold a conductive powder, vibrationamplifying means in approximately spiral, concentric, or turned-aroundform which is fixed to the approximately flat container, with at least apart thereof remaining unfixed, means to vibrate the above-mentionedapproximately flat container, and means to cause a catalytic substanceto physically adhere to the above-mentioned conductive powder.

The present invention is also directed to a vibrating apparatus whichincludes means to vibrate a conductive powder and a means to amplifyvibration.

The present invention is directed to a process for production of acatalyst electrode capable of gas diffusion, the above-mentioned processincluding a step of causing a catalytic substance to adhere to thesurface of a conductive powder by physical vapor deposition whilevibrating the above-mentioned conductive powder together with vibrationamplifying means on a vibrating plane, thereby giving conductivecatalyst particles, and a step of preparing a catalyst electrode capableof gas diffusion which contains the thus obtained conductive catalystparticles.

As mentioned above, the process of the present invention includes a stepof causing a catalytic substance to adhere to the surface of aconductive powder by physical vapor deposition while vibrating theabove-mentioned conductive powder together with vibration amplifyingmeans on a vibrating plane. The advantage of this step is that theabove-mentioned conductive powder is thoroughly mixed by vibrationwithout staying at one place on the above-mentioned vibrating plane.Mixing in this manner causes the particles of the conductive powder, inoutside layers as well as inside layers, to be exposed, so that thecatalytic substance uniformly adheres to the above-mentioned conductivepowder.

As mentioned above, the process of the present invention includes a stepof causing a catalytic substance to adhere to the surface of aconductive powder by physical vapor deposition. The advantage of thisstep is that a catalytic substance with good crystallinity adheres at alow temperature only to the surface of the above-mentioned conductivepowder effectively without entering pores existing in the surface of theabove-mentioned conductive powder. The resulting conductive catalystparticles exhibit good catalytic actions even when used in a smallamount. In addition, they provide a sufficient area for contact betweenthe above-mentioned catalytic substance and gas. This implies that theabove-mentioned catalytic substance has a large specific surface areacontributing to reaction and hence shows enhanced catalytic capability.

A process for producing conductive catalyst particles by causing acatalytic substance such as platinum to adhere to the surface ofconductive powder such as carbon by physical vapor deposition such assputtering while vibrating the conductive powder, wherein the conductivepowder undergoes vibration together with vibration amplifying means suchas balls arranged on a vibrating plane according to an embodiment. Aprocess for producing a catalyst electrode capable of gas diffusionwhich has a step of incorporating the conductive catalyst particles intoit according to an embodiment. An apparatus for producing conductivecatalyst particles which includes means to vibrate conductive powder,means for physical vapor deposition to cause a catalytic substance toadhere to the surface of the conductive powder, and means to amplifyvibration according to an embodiment.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Invention and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic sectional view showing an apparatus for producingconductive catalyst particles according to an embodiment of the presentinvention.

FIG. 2 is a schematic diagram showing the container used in theapparatus for production according to an embodiment of the presentinvention.

FIG. 3 is a schematic sectional view showing the apparatus forproduction according to an embodiment of the present invention.

FIGS. 4A and 4B are schematic sectional views showing the vibratingapparatus used in the apparatus for production according to anembodiment of the present invention.

FIGS. 5A and 5B are schematic sectional views showing another vibratingapparatus used in the apparatus for production according to anembodiment of the present invention.

FIGS. 6A and 6B are schematic sectional views showing further anothervibrating apparatus used in the apparatus for production according to anembodiment of the present invention.

FIG. 7 is a schematic diagram showing the structure of the fuel cellprovided with the catalyst electrode capable of gas diffusion which isobtained by the process for producing conductive catalyst particlesaccording to an embodiment of the present invention.

FIG. 8 is a schematic diagram showing the structure of the hydrogengenerating apparatus provided with the catalyst electrode capable of gasdiffusion which is obtained by the process according to an embodiment ofthe present invention.

FIGS. 9A and 9B are diagrams showing the structure of the fullerenehydroxide (as an example of fullerene derivatives) which can be usedaccording to an embodiment of the present invention.

FIGS. 10A and 10B are schematic diagrams showing an example of thefullerene derivatives according to an embodiment of the presentinvention.

FIGS. 11A and 11B are schematic sectional view showing a conductivecatalyst particle produced by a conventional method which consists of acarbon article and platinum particles supported thereon.

FIG. 12 is a schematic sectional view showing an apparatus for producingconductive catalyst particles according to the invention as disclosed inJapanese Patent Application No. 2000-293517.

FIGS. 13A and 13B are schematic sectional views showing the conductivecatalyst particles obtained by the production process according to anembodiment of the present invention.

DETAILED DESCRIPTION

The invention will be described in more detail with reference to theembodiments thereof.

The physical vapor deposition mentioned above according to an embodimentshould preferably be carried out by a sputtering process which employsthe above-mentioned catalytic substance as the target. The sputteringprocess permits easy production with high productivity and forms filmsatisfactorily.

The sputtering process mentioned above may be replaced by a pulsed laserdeposition process, which permits easy control in film formation andforms film satisfactorily or other suitable process.

The sputtering process or the pulsed laser deposition process permits acatalytic substance with good crystallinity to adhere at a lowtemperature only to the surface of the conductive powder (withoutentering pores existing in the surface of the conductive powder).Therefore, the resulting conductive catalyst particles produce goodcatalytic action even when used in small quantities. Moreover, theyprovide a sufficient area for contact between the catalytic substanceand gas. This makes the catalytic substance possess a large specificsurface area contributing to reaction, which leads to enhanced catalyticcapability.

The means for physical vapor deposition may be at least one of thevacuum deposition means, the sputtering apparatus, the pulsed laserdeposition apparatus and the like.

The process according to an embodiment of the present invention, whichincludes the step of causing the catalytic substance to adhere to thesurface of conductive powder, has the advantage over the process offorming noble metal film on a carbon sheet by sputtering, which isdisclosed in a Japanese Translations of PCT for Patent No. Hei11-510311. In this regard, it is believed that the present inventionmakes the catalytic substance have a larger specific surface areacontributing to reaction, and this leads to enhanced catalyticcapabilities.

FIG. 1 is a schematic sectional view showing the apparatus for producingconductive catalyst particles according to an embodiment the presentinvention.

The apparatus shown in FIG. 1 is designed such that the surface of theconductive powder 1 is coated with the catalytic substance by physicalvapor deposition such as sputtering which employs the catalyticsubstance as the target 2. This apparatus employs one or more members,such as one or more spherical-shaped members including balls 3 or othersuitable material with a smooth surface as the vibration amplifyingmeans. In an embodiment, the conductive powder 1 and the balls 3 aremixed together, and the resulting mixture is placed on the vibratingplane 20 in the container 4 having an approximately flat bottom. Thevibrating plane 20 should preferably be vibrated by a transducer 5 suchas an electromagnetic coil or a supersonic horn. The vibrating apparatus21 constructed as mentioned above moves the balls 3, thereby mixing themwith the conductive powder 1 and causing the conductive powder 1 to flowwithout staying at one place on the vibrating plane. Moreover, themixing by the balls causes the particles of the conductive powder 1, inoutside layers as well as inside layers, to be exposed, so that thecatalytic substance uniformly adheres to the entire conductive powder 1.

In an embodiment, the balls 3 used for mixing preferably include ceramicballs, metallic balls and/or the like and range from about 1 mm to about10 mm in diameter. It should be appreciated that any suitable materialcan be utilized in any suitable size and shape.

FIG. 2 is a partly enlarged schematic diagram showing the container 4containing the conductive powder and the balls as the vibrationamplifying means. As shown in FIG. 2, the conductive powder and theballs should be mixed together such that the total area A of the ballsaccount for about 30% to about 80% of the area S in which the conductivepowder is distributed. If this ratio is excessively small, satisfactorymixing is not achieved. Conversely, if this ratio is excessively large(with a small proportion of the conductive powder 1), sputtering foradhesion of the catalytic substance is poor in efficiency (which leadsto a low productivity of the catalyst particles carrying the catalyticsubstance).

FIG. 3 is a partly enlarged schematic sectional view showing thecontainer 4 which holds the conductive powder 1 and the balls 3 as thevibration amplifying means. As shown in FIG. 3, the balls 3 shouldpreferably have a diameter (R) which is equivalent to about 10% to about70% of the layer thickness (t) of the conductive powder 1. The balls 3with a diameter outside this range are undesirable as in the case ofsurface ratio mentioned above.

The transducer 5 should apply vibration to the conductive powder 1 andthe balls 3 (as vibration amplifying means) at a frequency of about 5 Hzto about 200 Hz and an amplitude of ±(0.5 to 20) mm for their thoroughmixing. (This applies to other embodiments mentioned later.)

Sputtering in combination with vibration permit the catalytic substanceto adhere more uniformly to the surface of the conductive powder thansputtering without vibration. With the ball diameter smaller than about1 mm or larger than about 10 mm, the frequency less than about 5 Hz ormore than about 200 Hz, and the amplitude less than about ±0.5 mm,vibration does not shake the conductive powder strongly but permits itto stay on the bottom of the container. In this situation, uniform filmformation is unattainable. Moreover, vibration with an amplitude inexcess of about 20 mm may force out the conductive powder, which leadsto a decreased yield.

In the production process according to an embodiment of the presentinvention, the above-mentioned balls as the vibration amplifying meansmay be replaced by a flat object of wire in a form resembling a spiral,concentric circle, or turned-around pattern. This flat object isinstalled on the bottom of the container in such a way that at leastpart of it remains unfixed (for three-dimensional movement or vibrationwithout restrictions). In operation, the conductive powder is placed onthis flat object which is allowed to vibrate.

FIGS. 4A and 4B are schematic sectional views showing the vibratingapparatus according to an embodiment of the present invention. Thisapparatus is provided with the flat object 6 (as the vibrationamplifying means) made of wire in spiral form.

FIGS. 5A and 5B are schematic sectional views showing the vibratingapparatus according to an embodiment of the present invention. Thisapparatus is provided with the flat object 7 (as the vibrationamplifying means) made of wire in concentric form (with each sectionarranged in the radial direction).

FIGS. 6A and 6B are schematic sectional views showing the vibratingapparatus according to an embodiment of the present invention. Thisapparatus is provided with the flat object 8 (as the vibrationamplifying means) made of wire in turned-around pattern.

All the apparatuses shown in FIGS. 4A and 4B through 6A and 6B cause theconductive powder 1 to vigorously flow by vibration owing to any of theflat objects 6, 7, and 8, because the flat object is installed on thebottom of the container 4, with part of it remaining unfixed, and theconductive powder 1 is placed on and vibrated by the flat object. Duringvibration, the flat object retains its shape. While being vibrated, theconductive powder 1 undergoes physical vapor deposition such assputtering. In this way, the conductive powder 1 in the container 4 isthoroughly mixed and uniformly coated with the catalytic substance.

In an embodiment, for the flat object, such as in a spiral pattern,concentric pattern, turned-around pattern or the like, to produce thedesired effect, it should be formed from a wire having a diameter ofabout 1 mm to 10 about mm and it should have an outside diameter smallerby about 5 mm than the inside diameter of the container. In addition,the pattern should be formed such that adjacent wires are about 5 mm toabout 15 mm apart. The flat object not meeting these conditions does notachieve the thorough mixing of the conductive powder 1 and hence doesnot achieve the efficient coating with the catalytic substance.

The above-mentioned flat object formed from wire should preferably havea thickness equivalent to about 10% to about 70% of the thickness of thelayer of the conductive powder.

The production process according to an embodiment of the presentinvention affords the conductive catalyst particles in which thecatalytic substance 18 adheres only to the surface of the conductivepowder 1, effectively without entering pores (not shown) existing in thesurface of the conductive powder 1, as shown in FIG. 13A. Therefore, theconductive catalyst particles thus obtained produce the desirablecatalytic action even when used in small quantities. Moreover, they havea sufficiently large area for contact between the catalytic substance 18and gas and hence the catalytic substance 18 has a large specificsurface area that contributes to reaction. This leads to the enhancedcatalytic performance.

In other words, physical vapor deposition yields conductive catalystparticles in which the catalytic substance 18 adheres to the entiresurface of the conductive powder 1, as shown in FIG. 13A. This formationpermits the catalytic substance 18 in a small amount to perform a goodcatalytic function. This formation provides a sufficient area forcontact between the catalytic substance 18 and gas, thereby increasingthe specific surface area of the catalytic substance 18 whichcontributes to reaction and hence improving the catalytic performance.

The production process according to an embodiment of the presentinvention may be modified such that the conductive powder 1 is coatedwith the ionic conductor 19 and the catalytic substance 18 is depositedon the ionic conductor 19 by physical vapor deposition mentioned above,as shown in FIG. 13B. Unlike the conventional process, the modifiedprocess, in which the catalytic substance 18 is deposited by physicalvapor deposition, obviates the necessity of performing thermal treatmentto improve the crystallinity of the catalytic substance 18 and permitsthe catalytic substance 18 to deposit without adverse effect on theperformance of the ionic conductor 19.

Either of the conductive catalyst particles shown in FIGS. 13A and 13Bshould preferably carry the catalytic substance 18 in an amountequivalent to about 10 wt % to about 1000 wt % of the conductive powder1, so that they perform good catalytic action and exhibit goodconductivity. In an embodiment, the catalytic substance 18 includes anoble metal, such as Pt, Ir, Rh and/or the like.

The conductive powder 1, in an embodiment, should preferably have anelectrical resistance lower than about 10⁻³Ω·m. It may be at least anyone selected from carbon, ITO SnO₂ and the like. ITO stands for IndiumTin Oxide. It is an conductive oxide obtained by doping indium oxidewith tin.

Carbon as the conductive powder 1 includes a specific surface arealarger than about 300 m²/g in an embodiment. Any carbon not meeting thisrequirement would impair the characteristic properties of the conductivecatalyst particles.

In addition, carbon as the conductive powder 1 should preferably havegood gas permeability which is indicated by an oil absorption valuehigher than about 200 mL/100 g. Good gas permeability is important forthe catalyst electrode capable of gas diffusion which is made with theconductive catalyst particles.

The conductive catalyst particles produced by the process according toan embodiment of the present invention may be formed alone into acatalyst layer by pressing or the like. However, it may also be formedinto a film by binding with a resin. The resulting film is composed of aporous gas-permeable current collector and the conductive catalystpowder firmly adhering thereto. Such a film is desirable for theproduction of the catalyst electrode capable of gas diffusion.

The catalyst electrode capable of gas diffusion may be composed of theconductive catalyst particles almost alone. Alternatively, it maycontain, in addition to the conductive catalyst particles, auxiliarycomponents such as resin to bind them. In the latter case, the auxiliarycomponents include a water-repellent resin such as fluorocarbon resin(which contributes to binding performance and water drainage), apore-forming agent such as CaCO₃ (which contributes to gaspermeability), and an ionic conductor (which contributes to the mobilityof protons and the like). Moreover, the conductive catalyst particlesshould preferably be supported on a porous gas-permeable currentcollector (such as carbon sheet).

The catalyst electrode capable of gas diffusion which is produced by theprocess according to an embodiment of the present invention may beapplied to electrochemical devices such as fuel cells and hydrogengenerating apparatus.

In an electrical device consisting basically of a first electrode, asecond electrode, and an ionic conductor held between them, the catalystelectrode capable of gas diffusion may be used as at least the first ofthe two electrodes.

To be more specific, the catalyst electrode capable of gas diffusion maybe applied to electrochemical devices in which at least one of the twoelectrodes is a gas electrode.

FIG. 7 shows a typical example of fuel cells in which the catalystelectrode capable of gas diffusion is used. The catalyst layer 9 in FIG.7 is formed from the above-mentioned conductive catalyst particles aloneor in combination with an ionic conductor, a water-repellent resin (suchas fluorocarbon resin), and a pore-forming agent (such as CaCO₃). Theconductive catalyst particles are those which are obtained by theprocess of the present invention in an embodiment, and they are composedof a conductive powder (such as carbon powder) and a catalytic substance(such as platinum) adhering only to the surface thereof. The catalystlayer 9 and its adjacent carbon sheet 10 (as a porous gas-permeablecurrent collector) constitute the porous catalyst electrode capable ofgas diffusion, which is the one obtained by the process of the presentinvention. In a narrow sense, the catalyst layer 9 alone may be calledthe catalyst electrode capable of gas diffusion. An ionic conductor 11is held between the first and second electrodes which are the catalystelectrodes capable of gas diffusion.

The fuel cell shown in FIG. 7 has a negative electrode (fuel or hydrogenelectrode) 13 connected to a terminal 12, and a positive electrode(oxygen electrode) 15 connected to a terminal 14, with an ionicconductor 11 held between them. The negative electrode (and optionallythe positive electrode also) is the catalyst electrode capable of gasdiffusion which is obtained by the process according to an embodiment ofthe present invention. When the fuel cell is in operation, hydrogenpasses through the hydrogen passage 16 adjacent to the negativeelectrode 13. While passing through the passage 16, hydrogen (fuel)gives rise to hydrogen ions. These hydrogen ions, together with hydrogenions generated by the negative electrode 13 and the ionic conductor 11,migrate to the positive electrode 15, at which they react with oxygen(air) passing through the oxygen passage 17. Thus, there is obtained anelectromotive force as desired.

The fuel cell just mentioned above has as the first and secondelectrodes the catalyst electrode capable of gas diffusion which isobtained by the process of the present invention. Therefore, theelectrodes perform good catalytic action and provide a sufficient areafor contact between the catalytic substance and gas (hydrogen), allowingthe catalytic substance to have a large specific surface areacontributing to reaction and to exhibit improved catalytic performance.This imparts good output characteristics to the fuel cell. Anotheradvantage is a high hydrogen ion conductivity, which results from thefact that dissociation of hydrogen ions takes place in the negativeelectrode 13 and the hydrogen ions supplied from the negative electrode13 migrate to the positive electrode 15 while dissociation of hydrogenions is taking place in the ionic conductor 11.

FIG. 8 shows a typical example of hydrogen producing apparatus in whichthe first and second electrodes are the catalyst electrode capable ofgas diffusion which is obtained by the process of the present invention.

Reactions at each electrode take place as follows.

at positive electrode: H₂O→2H⁺+½O₂+2e⁻

at negative electrode: 2H⁺+2e⁻→H₂⇑

Theoretically, these reactions generate a voltage higher than 1.23 V.

The catalyst layer 9′ in FIG. 8 is formed from the above-mentionedconductive catalyst particles alone or in combination with an ionicconductor, a water-repellent resin (such as fluorocarbon resin), and apore-forming agent (such as CaCO₃). (The conductive catalyst particlesare those which are obtained by the process of the present invention,and they are composed of a conductive powder (such as carbon powder) anda catalytic substance (such as platinum) adhering only to the surfacethereof. The catalyst layer 9′ and its adjacent carbon sheet 10′ (as aporous gas-permeable current collector) constitute the porous catalystelectrode capable of gas diffusion, which is the one obtained by theprocess of the present invention. An ionic conductor 11′ is held betweenthe first and second electrodes which are the gas-permeable catalystelectrodes.

When in operation, this hydrogen producing apparatus is supplied withsteam or steam-containing air through an inlet close to the positiveelectrode 15′. The steam or steam-containing air is decomposed by thepositive electrode 15′ into electrons and protons (hydrogen ions). Thegenerated electrons and protons migrate to the negative electrode 13′,by which they are converted into hydrogen gas. In this way there isobtained hydrogen gas as desired.

The hydrogen-producing apparatus mentioned above is characterized inthat at least the first of its two electrodes is the catalyst electrodecapable of gas diffusion which is obtained by the process according toan embodiment of the present invention. This constitution permitsprotons and electrons to migrate smoothly through the positive electrode15′ so that hydrogen is generated at the negative electrode 13′.

As mentioned above, the catalyst electrode capable of gas diffusionwhich is obtained by the process of the present invention employs anionic conductor, or the electrochemical device employs an ionicconductor held between the first and second electrodes. The ionicconductor can include any suitable material. For example, the ionicconductor includes NAFION (perfluorosulfonic acid resin made by DUPONT)fullerene derivative such as fullerenol, polyfullerene hydroxide, and/orother suitable materials.

Fullerenol is fullerene having hydroxyl groups attached thereto, asshown in FIGS. 9A and 9B. The first synthesis of this compound wasreported in 1992 by Chiang et al. (Chiang, L. Y.; Swirczewski, J. W.;Hsu, C. S.; Chowdhury, S. K.; Cameron, S.; and Creegan, K., J. Chem.Soc, Chem. Commun. 1992, 1791)

In general, fullerenol can be formed into an aggregate so that hydroxylgroups in adjacent fullerenol molecules react with each other as shownin FIG. 10A (in which a circle denotes a fullerene molecule). Theresulting aggregate (as a macroscopic aggregate) exhibits desirableproton conducting characteristics or dissociation of H⁺ from thephenolic hydroxyl group in the fullerenol molecule.

Moreover, the fullerenol as the ionic conductor may be replaced by anaggregate of fullerene having more than one OSO₃H group. This fullerenederivative, in which OH groups are replaced by OSO₃H groups, ishydrogensulfate-esterified fullerenol, as shown in FIG. 10B. See, forexample, Chiang. L. Y.; Wang, L. Y.; Swirczewski, J. W.; Soled, S.; andCameron, S., J. Org. Chem. 1994, 59, 3960. Thehydrogensulfate-esterified fullerenol may have OSO₃H groups alone in onemolecule or both OSO₃H groups and hydroxyl groups in one molecule.

An aggregate formed from a large number of molecules of fullerenol orhydrogensulfate-esterified fullerenol exhibits proton conductivity asthe property of a bulk material. This proton conductivity directlyinvolves migration of protons derived from a large number of hydroxylgroups (which originally exist in molecules) and OSO₃H groups.Therefore, this proton conductivity does not need to rely on hydrogen(or protons) originating from steam molecules supplied from theatmosphere. In other words, the proton conductivity manifests itselfwithout supply of water from the outside or absorption of moisture fromthe air. This permits continuous use even in a dry atmosphere regardlessof the environment.

A probable reason for the significant proton conductivity of thefullerene derivatives mentioned above is that fullerene as the baseshows conspicuous electrophilicity which greatly promotes theelectrolytic dissociation of hydrogen ions from the hydroxyl groups aswell as from the highly acidic OSO₃H groups. In addition, the protonconductivity can be effectively increased because each fullerenemolecule can accept a considerable number of hydroxyl groups and OSO₃Hgroups, so that the number of protons involved in conduction is verylarge per unit volume of the conductor.

The fullerenol and hydrogensulfate-esterified fullerenol mentioned aboveis composed mainly of carbon atoms arising from fullerene. Therefore,they are light in weight, stable in quality, and free of contaminants.In addition, fullerene itself is rapidly decreasing in production cost.The foregoing makes fullerene a nearly ideal carbonaceous materialstanding above other materials from the standpoint of resource,environment, and economy.

Other fullerene derivatives than mentioned above may also be used, whichhave any of —COOH, —SO₃H, —OPO(OH)₂ and the like in place of —OH and—OSO₃H.

The fullerenol or fullerene derivative mentioned above may be obtainedby treating fullerene powder with an acid or hydrolyzing fullerene. Anyknown process can attach desired groups to carbon atoms constitutingfullerene molecules.

The fullerene derivative used as the ionic conductor mentioned above maybe in the form of simple body (composed of fullerene derivative almostalone) or in the form of lump solidified with a binder.

The fullerene derivative may be made into a film by press-forming, sothat the resulting film serves as the ionic conductor held between thefirst and second electrodes. This ionic conductor in film form may bereplaced by the one which is obtained from a lump of fullerenederivative solidified with a binder. In this case, the ionic conductorhas a sufficient strength.

The binder mentioned above may be one or more than one kind of any knownpolymeric material capable of forming film. It should be used in anamount less than about 20 wt % of the ionic conductor; it willdeteriorate the conductivity of hydrogen ions if used in an amount morethan about 20 wt %.

The ionic conductor composed of the fullerene derivative and a binderpermits conduction of hydrogen ions in the same way as that composed ofthe fullerene derivative alone. In addition, owing to the film-formingpolymeric material contained therein, the former is stronger than thelatter (which is formed by compression from the fullerene derivative inpowder form). It is a flexible ion-conducting thin film (usually thinnerthan 300 μm) capable of blocking gas permeation.

The polymeric material mentioned above is not specifically restricted solong as it forms a film without inhibiting the hydrogen ion conductivity(by reaction with the fullerene derivative). It should preferably be onewhich is stable without electron conductivity. It includes, for example,polyfluoroethylene, polyvinylidene fluoride, polyvinyl alcohol and thelike, which are desirable for reasons given below.

Polyfluoroethylene is desirable because it permits the fullerenederivative to be formed into a strong thin film more easily with a lessamount than other polymeric materials. It produces its effect only witha small amount of about 3 wt %, preferably about 0.5 wt % to about 1.5wt %. It gives a thin film ranging from about 100 μm down to about 1 μmin thickness.

Polyvinylidene fluoride and polyvinyl alcohol are desirable because theygive an ion-conducting thin film with a remarkable ability to block gaspermeation. In an embodiment, they are used in an amount of about 5 wt %to about 15 wt %.

The above-mentioned polymeric materials may adversely affect theirfilm-forming performance if they are used in an amount less than thelower limit of the range mentioned above.

The thin film for the ionic conductor composed of the fullerenederivative and the binder may be obtained by any known film-formingprocess such as compression molding and extrusion molding.

The invention will be described in more detail with reference to thefollowing examples.

Example 1

An apparatus shown in FIG. 1 was assembled from a sputtering target, avibrator, and a container. The container was charged with a conductivepowder and balls. The sputtering target is a platinum disc 100 mm indiameter. The balls are stainless steel balls 3 mm in diameter. Theconductive powder is a carbon powder having a specific surface area of800 m²/g and an oil absorption value of 360 mL/100 g. The vibratorgenerates vibration with an amplitude of ±5 mm and a frequency of 36 Hz.

The container was charged with the carbon powder (1 g) and the stainlesssteel balls (35 g). Sputtering was carried out for 30 minutes while thecarbon powder and stainless steel balls were being vibrated by thevibrator, with the vacuum chamber supplied with argon (1 Pa) and thetarget activated by 400 W RF. After sputtering, it was found that thecarbon powder increased in weight to 1.66 g owing to deposition ofplatinum (0.66 g) thereon. This implies that the treated carbon powdercarries platinum as much as 40 wt %.

A carbon sheet was coated with a mixture of teflon binder and carbon(not carrying platinum) dispersed in a solvent such that the coatinglayer was 20 μm thick after drying. This coating was used as a layer toprevent spreading.

The platinum-carrying carbon powder, which was obtained as mentionedabove, was mixed with perfluorosulfonic acid resin (as a binder) andn-propyl alcohol (as an organic solvent). The resulting mixture wasapplied to that side of the carbon sheet on which the layer to preventspreading was formed. After drying, the coating layer was 10 μm thick.The thus obtained sheet was used in this example as the catalystelectrode capable of gas diffusion. A fuel cell as shown in FIG. 7 wasmade, in which the catalyst electrode capable of gas diffusion wasplaced on both sides of the ion-exchange membrane (of perfluorosulfonicacid resin). The resulting fuel cell was tested for output. The output(in terms of mW/cm²) achieved by the fuel cell in this example isregarded as the standard (100%) for relative values.

Example 2

An apparatus as shown in FIGS. 4A and 4B was assembled from a sputteringtarget, a vibrator, a container, and a device made of spirally woundwire. The wire is a stainless steel wire 1.6 mm in diameter. The wire iswound at a pitch of about 5 mm to about 10 mm. The outside diameter ofthe spiral is smaller by about 5 mm than the inside diameter of thecontainer. The conductive powder (carbon powder) mentioned above wasplaced on the device of spirally wound wire. Being not fixed to thebottom of the container, this device vibrated together with the carbonpowder during sputtering to coat the carbon powder with platinum.

A fuel cell as shown in FIG. 7 was made in the same way as in Example 1except that the carbon powder was coated with platinum by using theapparatus mentioned above. The obtained fuel cell was tested for output.The output was 120% of that in Example 1.

Example 3

A fuel cell shown in FIG. 7 was made in the same way as in Example 1except that the carbon powder (which carries platinum) was replaced bythe one which has an oil absorption value of 150 mL/100 g. The obtainedfuel cell was tested for output. The output was 65% of that in Example1.

Example 4

A fuel cell shown in FIG. 7 was made in the same way as in Example 1except that the carbon powder (which carries platinum) was replaced bythe one which has a specific surface area of 200 m²/g. The obtained fuelcell was tested for output. The output was 65% of that in Example 1.

COMPARATIVE EXAMPLE 1

Platinum coating on the carbon powder was accomplished by the liquidphase method in the following manner. The carbon powder was immersed inan aqueous solution of hexaamineplatinum(IV) chloride([Pt(IV)(NH₃)₆]Cl₄) containing 10 g/L of platinum at room temperaturefor 1 hour. Then, the carbon powder was washed and heated at 180° C. ina hydrogen stream for reduction of the platinum salt. Thus there wasobtained a platinum-carrying carbon powder. A fuel cell shown in FIG. 7was made in the same way as in Example 1 except that platinum-carryingcarbon powder was used as the conductive catalyst particles. The thusobtained fuel cell was tested for output. The output was 50% of that inExample 1.

COMPARATIVE EXAMPLE 2

A fuel cell shown in FIG. 7 was made in the same way as in Example 1except that the carbon powder was coated with platinum by sputteringwithout balls (as the vibration amplifying means) and the carbon powderwas not vibrated. The thus obtained fuel cell was tested for output. Theoutput was 30% of that in Example 1.

COMPARATIVE EXAMPLE 3

A fuel cell shown in FIG. 7 was made in the same way as in Example 1except that the carbon powder (as the conductive powder) was coated withplatinum by sputtering without balls in the vibrating apparatus (onlythe carbon powder was vibrated). The thus obtained fuel cell was testedfor output. The output was 60% of that in Example 1.

It is apparent from the foregoing that the production process accordingto an embodiment of the present invention permits uniform platinumcoating on carbon powder. This is because carbon powder (as a conductivepowder), together with balls (as a vibration amplifying means), isplaced in a vibrating container and is coated with platinum bysputtering (as physical vapor deposition) while they are being vibrated.Vibration in this manner keeps the carbon powder moving without settlingon the bottom of the container. The thus obtained platinum-coated carbonpowder (as conductive catalyst particles) is suitable for the catalystelectrode capable of gas diffusion which is built into a high-outputfuel cell.

The production process according to an embodiment of the presentinvention includes coating carbon powder with platinum (as a catalyticsubstance) by physical vapor deposition (sputtering). This processpermits platinum with good crystallinity to adhere only to the surfaceof the carbon powder at a low temperature. Therefore, the resultingconductive catalyst particles produce good catalytic action for itssmall amount. In addition, they provide a sufficient area for contactbetween platinum and gas, making platinum have a large specific surfacearea for reaction. This leads to improved catalytic performance. Thus,as a constituent of the catalyst electrode capable of gas diffusion,they contribute to a high-output fuel cell.

The fuel cell in Example 3 has a low output on account of poor gaspermeability resulting from the carbon powder to carry platinum havingan oil absorption value of 150 mL/100 g which is lower than 200 mL/100 gspecified in the present invention.

The fuel cell in Example 4 has a low output on account of unsatisfactoryconductive catalyst particles resulting from the carbon powder to carryplatinum having a specific surface area of 200 m²/g which is smallerthan 300 m²/g specified in the present invention.

The fuel cell in Comparative Example 1 is poor in catalyst efficiencybecause platinum coating on carbon powder was carried out by the liquidphase method which causes platinum to exist as unstable spheres on thesurface of carbon powder.

The fuel cell in Comparative Example 2 has a low output on account ofthe uneven platinum coating on carbon powder which results fromsputtering without vibration and balls (as the vibration amplifyingmeans). Sputtering in this manner merely deposits platinum on carbonpowder existing in the surface layer in the container.

The fuel cell in Comparative Example 3 has a low output on account ofthe uneven platinum coating on carbon powder which results fromsputtering without balls (as the vibration amplifying means). Sputteringin this manner does not uniformly deposit platinum on the entire carbonpowder in the container.

The above-mentioned embodiments may be variously modified as followswithin the scope of the present invention.

For example, the balls or the spirally wound device used as thevibration amplifying means may be replaced by a device made of a wirewhich is concentrically wound as shown in FIGS. 5A and 5B or by a devicemade of a wire which is turned-around as shown in FIGS. 6A and 6B. Bothof them produce good results as in Example 2.

Moreover, the carbon powder used as the conductive powder may bereplaced by ITO SnO₂ and/or the like.

Further, the catalyst electrode capable of gas diffusion which isproduced according to the present invention is assumed to be used forthe fuel cell as explained above. However, it may also be used for thehydrogen-producing apparatus in which the reaction in the fuel cell isreversed.

The process according to an embodiment of the present invention includescoating the conductive powder with the catalytic substance by physicalvapor deposition by vibrating it together with the vibration amplifyingdevice on the vibrating plane. Vibration in this manner ensures thoroughmixing up and down, without the conductive powder partially stayingstill on the vibrating plane. The result of thorough mixing is uniformcoating of the catalytic substance on the conductive powder.

The advantage of the physical vapor deposition used to coat the surfaceof the conductive powder with the catalytic substance is that thecatalytic substance with good crystallinity adheres at a low temperatureonly to the surface of the conductive powder. The resulting conductivecatalyst particles produce a good catalytic action even when used insmall quantities. In addition, they provide a sufficient area forcontact between the catalytic substance and gas. That is, they have alarge specific surface area for reactions and hence exhibit improvedcatalytic performance.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present invention andwithout diminishing its intended advantages. It is therefore intendedthat such changes and modifications be covered by the appended claims.

1. A process for producing conductive catalyst particles, the processcomprising causing a catalytic substance to adhere to a surface of aconductive powder by physical vapor deposition while vibrating theconductive powder, wherein the conductive powder undergoes vibrationtogether with vibration amplifying means arranged on a vibrating plane.2. The process for producing conductive catalyst particles as defined inclaim 1, wherein the conductive powder and the vibration amplifyingmeans are vibrated by a vibrator at a frequency that ranges from about 5Hz to about 200 Hz.
 3. The process for producing conductive catalystparticles as defined in claim 1, wherein the physical vapor depositionincludes a sputtering process that employs the catalytic substance as atarget.
 4. The process for producing conductive catalyst particles asdefined in claim 1, wherein the physical vapor deposition includes apulsed laser deposition process.
 5. The process for producing conductivecatalyst particles as defined in claim 1, wherein the conductive powderis coated with the catalytic substance in a ratio of about 10 wt % toabout 1000 wt %.
 6. The process for producing conductive catalystparticles as defined in claim 1, wherein the catalytic substanceincludes a noble metal selected from the group consisting of Pt, Ir, Rhand combinations thereof.
 7. The process for producing conductivecatalyst particles as defined in claim 1, wherein the conductive powderis coated with the catalytic substance by the physical vapor depositionprocess after it has been coated with an ionic conductor.
 8. The processfor producing conductive catalyst particles as defined in claim 1,wherein the conductive powder includes an electric resistance lower thanabout 10⁻³Ω·m.
 9. The process for producing conductive catalystparticles as defined in claim 1, wherein the conductive powder isselected from the group consisting of carbon, Indium Tin Oxide, aconductive oxide obtained by doping indium oxide with tin, and SnO₂. 10.The process for producing conductive catalyst particles as defined inclaim 9, wherein the carbon has a specific surface area greater thanabout 300 m²/g.
 11. The process for producing conductive catalystparticles as defined in claim 9, wherein the carbon has an oilabsorption value greater than about 200 mL/100 g.
 12. A process forproducing a catalyst electrode capable of gas diffusion, the processcomprising the steps of: causing a catalytic substance to adhere to asurface of a conductive powder by physical vapor deposition whilevibrating the conductive powder together with vibration amplifying meanson a vibrating plane, thereby producing one or more conductive catalystparticles and preparing the catalyst electrode with the conductivecatalyst particles.
 13. The process for producing a catalyst electrodecapable of gas diffusion as defined in claim 12, wherein the conductivepowder and the vibration amplifying means are vibrated by a vibrator ata frequency that ranges from about 5 Hz to about 200 Hz.
 14. The processfor producing a catalyst electrode capable of gas diffusion as definedin claim 12, wherein the physical vapor deposition sputtering processemploys the catalytic substance as a target.
 15. The process forproducing a catalyst electrode capable of gas diffusion as defined inclaim 12, wherein the physical vapor deposition includes a pulsed laserdeposition process.
 16. The process for producing a catalyst electrodecapable of gas diffusion as defined in claim 12, wherein the conductivepowder is coated with the catalytic substance in a ratio that rangesfrom about 10 wt % to about 1000 wt %.
 17. The process for producing acatalyst electrode capable of gas diffusion as defined in claim 12,wherein the catalytic substance is a noble metal selected from the groupconsisting of Pt, Ir, Rh and combinations thereof.
 18. The process forproducing a catalyst electrode capable of gas diffusion as defined inclaim 12, wherein the conductive powder is coated with the catalyticsubstance by the physical vapor deposition process after it has beencoated with an ionic conductor.
 19. The process for producing a catalystelectrode capable of gas diffusion as defined in claim 12, wherein theconductive powder is one which has an electric resistance lower thanabout 10⁻³Ω·m.
 20. The process for producing a catalyst electrodecapable of gas diffusion as defined in claim 12, wherein the conductivepowder is selected from the group consisting of carbon, Indium TinOxide, a conductive oxide obtained by doping indium oxide with tin, SnO₂and combinations thereof.
 21. The process for producing a catalystelectrode capable of gas diffusion as defined in claim 20, wherein thecarbon a specific surface area greater than about 300 m²/g.
 22. Theprocess for producing a catalyst electrode capable of gas diffusion asdefined in claim 20, wherein the carbon has an oil absorption valuegreater than about 200 mL/100 g.
 23. The process for producing acatalyst electrode capable of gas diffusion as defined in claim 12,wherein the conductive catalyst particles are bound with a resin. 24.The process for producing a catalyst electrode capable of gas diffusionas defined in claim 12, wherein the conductive catalyst particles areattached onto a current collector.